Two high-ranking members of Congress are accusing the Justice Department of a cover-up related to a federal operation that reportedly sent American guns to violent Mexican drug gangs.

Citing private testimony from the acting head of the Bureau of Alcohol, Tobacco, Firearms, and Explosives, Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee, and Sen. Chuck Grassley, R-Iowa, ranking member of the Senate Judiciary Committee, told Attorney General Eric Holder in a letter dated Tuesday that acting ATF director Kenneth Melson met with the House panel's staff on July 4 and reported that Justice had concealed information about the operation, including the involvement of the FBI and Drug Enforcement Administration.

The operation, Project Gunrunner, has drawn scrutiny amid allegations that, rather than stanch the export of guns from the U.S. to Mexico, it has instead facilitated sales to traffickers. Issa and Grassley are raising the prospect that the purported scandal could have spread much wider and higher than the Obama administration has acknowledged.

Melson has reportedly been under pressure to resign. ATF agents told the Oversight Committee last month that they received orders to allow small-time dealers to operate in efforts to ensnare major operators, which led to drug cartels across the border receiving firearms unfettered by U.S. law enforcement.

The controversy flared in December when authorities tied two weapons at the scene of a border patrol official's shooting death to Operation Fast and Furious, an initiative under Project Gunrunner.

According to the letter, obtained Wednesday by National Journal, Melson took part in the interview with his personal counsel, instead of the Justice and ATF counsel who was originally scheduled to accompany him on July 13.

Department officials, Issa and Grassley said, "sought to limit and control his communications with Congress.

"Knowing what we know so far, we believe it would be inappropriate to make Mr. Melson the fall guy in an attempt to prevent further congressional oversight," Issa and Grassley wrote.

"The evidence we have gathered raises the disturbing possibility that the Justice Department not only allowed criminals to smuggle weapons but that taxpayer dollars from other agencies may have financed those engaging in such activities," they said. "While this is preliminary information, we must find out if there is any truth to it."

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